This application is based on patent application Hei. 11-280180 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to a piezoelectric actuator suitable for moving a driven member such as a two-dimensional moving table, a taking lens of a camera, a projection lens of an over-head projector, a lens of a binocular, and so on.
2. Description of the Related Art
In an impact type piezoelectric actuator, a driven member is movably coupled with a rod shaped driving member in an axial direction thereof. An end of the driving member is fixed on an end of a piezoelectric device in a manner so that the polarization of the piezoelectric device coincides with the axial direction of the driving member. For example, U.S. Pat. No. 5,589,723 shows a conventional impact type piezoelectric actuator used for moving a taking lens of a camera.
A basic configuration of the conventional impact type piezoelectric actuator is shown in FIG. 20. The piezoelectric actuator 100 comprises a rod shaped driving member 101, a driven member 102, a lamination type piezoelectric device 103 and a driving circuit 104. The driven member 102 is coupled with the driving member 101 by a predetermined friction force. When a dynamic force larger than the friction force is applied to the driven member 102, it can be moved in an axial direction relative to the driving member 101. An object to be moved such as a taking lens is connected to the driven member 102. The piezoelectric device 103 is connected on a base end 101b of the driving member 101 in a manner so that the polarization of the piezoelectric device 103 coincides with the axial direction of the driving member 101. A pair of electrodes 103a and 103b are provided on both ends of the piezoelectric device 103. The first electrode 103a is connected to the driving circuit 104 and the second electrode 103b is grounded.
The driving circuit 104 is configured by a forward driving circuit 105 for moving the driven member 102 toward a top end 101a of the driving member 101 (hereinafter, this direction is called xe2x80x9cforwardxe2x80x9d), a backward driving circuit 106 for moving the driven member 102 toward the base end 101b of the driving member 101 (hereinafter, this direction is called xe2x80x9cbackwardxe2x80x9d) and a controller 107 for controlling the forward driving circuit 105 and the backward driving controller 106, alternatively.
The piezoelectric actuator 100 moves the driven member 102 relative to the driving member 101 by utilizing a difference of friction forces acting between the driving member 101 and the driven member 102 corresponding to the moving speed of the driving member 101. When the driving member 101 moves quickly, the friction force becomes smaller. Alternatively, when the driving member 101 moves slowly, the friction force becomes larger. For moving the driven member 102 forward, the driving member is slowly moved forward and quickly moved backward (forward driving). For moving the driven member 102 backward, the driving member is quickly moved forward and slowly moved backward (backward driving).
The forward driving circuit 105 is configured by a slow charger 105a and a quick discharger 105b. The backward driving circuit 106 is configured by a quick charger 106a and a slow discharger 106b. The slow charger 105a and the quick charger 106a respectively apply a driving voltage Vp to the piezoelectric device 103 in the polarization direction (or charge the piezoelectric device 103 in the polarization direction) for expanding the piezoelectric device 103 in the polarization direction (or in the axial direction of the driving device 101). The quick discharger 105b and the slow discharger 106b respectively apply a voltage to the piezoelectric device 103 in the opposite direction to the polarization (or discharges the electric charge in the piezoelectric device 103 by grounding the electrode 103a) for contracting the piezoelectric device 103.
A configuration of the slow charger 105a is shown in FIG. 21. As can be seen from FIG. 21, the slow charger 105a is configured by a constant current charging circuit, in which a zener diode ZD is connected in parallel with a fixed biasing circuit of a pnp type transistor Tr1. Resistors r1 and r2 serve as biasing resistors of the transistor Tr1. The zener diode ZD is connected in parallel with the resistor r2. Since the zener diode ZD restricts the base current of the transistor Tr1 at a predetermined constant value, voltage drop owing to the resistor r1 can be maintained at a predetermined value. The collector current of the transistor Tr1 can be restricted to be constant. As a result, the charging current for charging the piezoelectric device 103 is restricted, so that the forward moving speed of the driving member 101 is restricted.
Alternatively, another configuration shown in FIG. 22 is used as the slow charger 105a. As can be seen from FIG. 22, an npn transistor Tr2 is used instead of the parallel circuit of the resistor r2 and the zener diode ZD. The collector and the base of the transistor Tr2 are respectively connected to the emitter and the base of the transistor Tr1. The emitter of the transistor Tr2 is connected to an electric power supply and the driving voltage Vp is applied thereto. Since the voltage at the base of the transistor Tr1 is maintained at a constant value by the transistor Tr2, voltage drop owing to the resistor r1 can be maintained at a predetermined value. The collector current of the transistor Tr1 can be restricted to be constant.
The quick discharger 105b and the slow discharger 106b apply a voltage to the piezoelectric device 103 in a direction opposite to the polarization of the piezoelectric device 103, so that the electric charge stored in the piezoelectric device 103 is discharged. Thus, the expanded piezoelectric device 103 is contracted. In FIG. 20, the electric charge in the piezoelectric device 103 is discharged by grounding the electrode 103a. 
A configuration of the slow discharger 106b is shown in FIG. 23. As can be seen from FIG. 23, the slow discharger 106b is configured by a constant current discharging circuit, in which a zener diode ZD is connected between the base of an npn type transistor Tr3 and the ground. A resistor r4 restricts the discharging current. Since the zener diode ZD restricts the base voltage of the transistor Tr3 at a predetermined constant value, voltage drop owing to the resistor r4 can be maintained at a predetermined value. The emitter current corresponding to the discharging current flowing in the resistor r4 can be restricted to be a predetermined value. As a result, the discharging current for discharging the piezoelectric device 103 is restricted, so that the backward moving speed of the driving member 101 is restricted.
The controller 107 alternately switches the slow charger 105a and the quick discharger 105b in the forward driving operation. The controller 107 alternately switches the quick charger 106a and the slow discharger 106b in the backward driving operation.
When the slow charger 105a and the quick discharger 105b are alternately driven in the forward driving operation, the piezoelectric device 103 repeats the slow expansion and the quick contraction, alternately. By such operation, the driving member 101 repeats the forward slow movement and the backward quick movement. When the quick charger 106a and the slow discharger 106b are alternately driven in the backward driving operation, the piezoelectric device 103 repeats the quick expansion and the slow contraction, alternately. By such operation, the driving member 101 repeats the forward quick movement and the backward slow movement.
The impact type piezoelectric actuator is generally used for driving an optical system of a portable equipment such as a taking lens of a camera and a lens of a binocular. It is preferable to make the driving circuit of the actuator compact and simple with regard to the downsizing and the lightening of the equipment.
In the above-mentioned conventional piezoelectric actuator, the charging current and the discharging current are restricted by the constant current circuits, so that a number of elements constituting the circuit inevitably increases. It becomes difficult to make the configuration and size of the driving circuit simple and small.
Furthermore, in the conventional piezoelectric actuator 100, it is necessary to change the circuit constant of circuit elements constituting the constant current circuit for varying the moving speed of the driven member 102. In this case, a plurality of circuit elements and switching circuits having the different circuit constants are necessary, so that the downsizing of the driving circuit 104 becomes more difficult.
On the other hand, U.S. Pat. No. 5,969,464 shows an actuator having a plurality of piezoelectric devices respectively having different driving characteristics. For varying the moving speed of the driven member, one of the piezoelectric devices is selectively driven for moving the driven member at a different moving speed. This actuator, however, has a disadvantage that the downsizing of the actuator becomes difficult because the driving circuit becomes complex and the piezoelectric devices occupy the space. U.S. Pat. No. 5,768,016 shows another actuator in which pulses of the driving signal are thinned for varying the moving speed of the driven member. This actuator has a disadvantage that the moving speed of the driven member becomes unstable.
An object of this invention is to provide an impact type piezoelectric actuator by which a moving speed of a driven member can smoothly be varied and an apparatus using the piezoelectric actuator can be downsized.
A piezoelectric actuator in accordance with an aspect of this invention comprises a piezoelectric device, a driving member driven by the expansion and contraction of the piezoelectric device, a holder for holding the piezoelectric device and the driving member, a driven member coupled with the driving member by a friction force and movable along the axial direction of the driving member, and a driving apparatus (or driving circuit) for applying driving voltage to the piezoelectric device. An end of the driving member is fixed on an end of the piezoelectric device in polarization direction of the piezoelectric device.
The driving apparatus or the driving circuit comprises a first driver for charging and/or discharging the piezoelectric device, a second driver for charging and/or discharging the piezoelectric device substantially the same charging and discharging speed as those of the first driver, and a controller for modifying at least one of a first driving time period of the first driver and a second driving time period of the second driver for varying the moving speed of the driven member or for changing the moving direction of the driven member.
By such a configuration, since the charging speed for charging the piezoelectric device can substantially be the same as the discharging speed thereof, it is possible to provide a relatively large difference between an expansion speed of the piezoelectric device and a contraction speed thereof by modifying the first driving time period of the first driver and the second driving time period of the second driver. When the moving speed of the driving member owing to the expansion or contraction of the piezoelectric device is sufficiently faster, the dynamic force acting on the driving member becomes larger than the friction force acting between the driving member and the driven member, so that the driven member can be moved relative to the driving member.
Furthermore, since the driving apparatus or the driving circuit modifies the charging time period and/or discharging time period of the piezoelectric device, the driving apparatus or the driving circuit needs no constant current circuit used in the conventional piezoelectric actuator. Thus, the configuration of the driving apparatus or the driving circuit of the piezoelectric actuator can be made simple and small.
Still furthermore, since the driving speed of the driven member can be controlled by modifying at least one of the first driving time period of the first driver and the second driving time period of the second driver, the moving speed of the driven member can be varied smoothly.